Today jurors in the Boston Marathon bombing case may learn more about what prosecutors call Dzhokhar Tsarnaev's "hidden" side with testimony from the FBI about the accused bomber’s secret online life -- one that embraced radical extremism and sent out an ominous message about the marathon in 2012.

FBI Special Agent Steven Kimball will take the stand again this morning to be cross-examined about his testimony Monday, which centered around Tsarnaev’s purported obsession with violent jihadi views espoused on the secret Twitter account @Al_Firdausi, a profile with tweets that praise an American al Qaeda cleric, Anwar al Awlaki, and declared hope for “victory over kfur [nonbelievers].”

"Dua [prayers of supplication] is truly the weapon of the believer, pray for the oppressed it is your duty," reads one tweet. Another reads, "It's our responsibility my brothers & sisters to Allah to ease the hardships of the oppressed and give us victory over kufr."

It’s through tweets like these that prosecutors hope to undercut the defense argument that “Jahar spent most of his time on the Internet doing things that teenagers do: Facebook, cars, girls," as his defense attorney Judy Clarke insisted in her opening statement last week. At the time of the bombings, Tsarnaev was 19 and his older brother Tamerlan was 26.

In a startling admission during opening statements last week, Clarke told jurors that her client was responsible of the crimes in contained in a federal indictment, saying, "it was him," but insisted, “Tamerlan led the way down Boylston Street."

“It was Tamerlan Tsarnaev who self-radicalized,” Clarke told the court. “It was Jahar who followed him."

The government hinted that it would reveal Dzhokhar Tsarnaev’s own alleged obsession with extremism in its opening statements.

“The defendant looked and acted like he was a typical young adult," Assistant U.S. Attorney William Weinreb said. “But the evidence will show that he wasn’t. He had a side to him that he kept hidden, even from his closest friends.”

FBI Special Agent Kimball laid out part of that life Monday, the third day of Tsarnaev’s trial. Tsarnaev maintained two Twitter accounts using email accounts that were linked to him and looped back to the University of Massachusetts/Dartmouth, where he was a student, Kimball testified.

On one, @J_Tsar, Tsarnaev used to tweet “Ain't no love in the heart of the city, stay safe people,” at 5:04 p.m. April 15, 2013, roughly two hours after he detonated the pressure cooker bomb that prosecutors said killed college student Lingzi Lu and eight-year-old Martin Richard. A video shown in court yesterday captures the moment when that bomb exploded and Dzhokhar hustled away in the chaos, pushing people out of the way as he fled the scene.

A tweet that he sent from that account a year earlier – on April 16, 2012 – Kimball told the court, could be interpreted as a threat against marathon organizers. It reads, “They will spend their money and they will regret it and then they will be defeated.”

Anwar al-Awlaki, to whom Tsarnaev allegedly encouraged his few online followers to listen, was an American high-profile member of al Qaeda who was linked to a number of domestic terror plots. Al-Awlaki was killed in a drone strike in Yemen in 2011.

Under cross examination, the defense attempted to paint a majority of Tsarnaev’s purported tweets as harmless. Many times, they said, he was quoting hip hop songs. The one that prosecutors indicated was an ominous tweet about the 2012 marathon was actually a line from the Quran, the defense said.

The second, secret Twitter account was under the name Ghuraba, which loosely translates as the Muslim word for strangers, and showed a picture of the Chechen capital Grozny – which the defense said had been misidentified by Kimball as the Muslim holy city of Mecca.

The indictment against Tsarnaev also accuses him of downloading jihadist material onto his computer, including writings by al-Awlaki and by another cleric who is referred to in the indictment as the "father of global jihad."

Those documents, the Twitter accounts, and the anti-American note the his lawyer admitted Tsarnaev scrawled on a dry-docked boat, wounded and bleeding, just before he was captured days after the bombing, prosecutors argue, are proof that Tsarnaev had "murder in his heart" when he dropped his bomb "right behind a row of children."

Jurors also saw for the first time today images of the note that Tsarnaev wrote on the boat. As previously seen in exclusive ABC News reports, the message blames the U.S. for “killing our innocent civilians” and says, “I can’t stand to see such evil go unpunished.”

Obtained by ABC News

This image, obtained exclusively by ABC News, appears to show the anti-American message allegedly written by Boston Marathon bombing suspect Dzhokhar Tsarnaev on the wall of a boat in which he hid just before being arrested last year.

Defense attorneys counter that their client was "drawn into his brother’s passion and plan," and that it was Tamerlan's "obsession" with violence in the Middle East that "led him [Dzhokhar] to Boylston Street.”

The defense also pointed out today that Tsarnaev apparently wasn't the only one with access to at least one of the accounts. The @j_tsar account appears to have been updated in October 2013, they said, long after Tsarnaev was in custody.